Got a strange issue. Fresh rebuilt trans on an 01 fbody. Car will not move if selector is put on OD or 3rd(D). Well it does move but you gota rev the **** out of it, and it barely moves a foot or 2. The car works perfect if I go down to reverse, manual 1st and or manual 2nd. Check wiring harness for continuity, and it all checks out. No check engine lights, vcm scanner shows everything functional, trans knows when it is in D4 D3 D2 D1. While in OD, if I enable the 2nd gear in the vcm scanner special functions, it does move forward while in OD, if I select 1st with vcm scanner while in OD it doesn't move. Check line pressure and it's steady at 90 psi, on reverse I cycles from around 190 psi to 110 psi or so(not sure if normal), all the other gears show very similar pressure [90 psi] when shifted into them. The builder is reputable this is the first time I have issues with any of his transmissions (over 40 transmissions) in 5 years or so. I bring trans down, he checks it and says everything is good. Install it, does the exact same thing. Go over all the basics again, everything checks out. At this point this thing has been at my shop for over a week and it needs to leave asap since I have more cars to finish so I watch a quick video on 4l60es and decide to dig into it myself. First thing I suspected was the forward sprag, it be damaged or installed incorrectly. Well its good and installed correctly. Teflon seals on the input drum are good, I test the drum with shop air and forward engages 3-4 engages etc etc . At the same time the trans builder shows up to tune a vehicle he had finished so he catches me disassembling his trans lol and decides to help me and we decide to just blindly swap the valve body at this point (we check for any stuck valves etc on the new unit). We put it back together. SAME THING LOL. At this point the only thing I can think of is maybe the seal at the bottom of the apply pistons on the input drum ( a green o ring seal) may be damaged and is causing issues? I didn't check that and quite frankly I didn't check the reactor side of the trans (the bottom half) because I was almost convinced it was the forward sprag from some quick light reading.

I am an experienced tuner and have been tuning and messing with this platform for over 14 years now, so when I say there's nothing wrong electronically/tuning wise, it is because I am 99.9% sure. Plus the car functioned perfectly fine before the trans being overhauled.
at this point I am using this experience to learn more about automatics and possibly start overhauling them myself so I am trying to figure this out as I think it is a mechanical issue w the trans. I'm tempted to just install a stock 60e to verify at this point. I have several trans shops I tune for and they all said forward sprag. One said check low roller sprag but not sure how that could be if it has reverse? And 2 of them said to check the o ring at bottom of input drum behind the apply pistons.

Any ideas? Thanks.

 

Did you use the Search entry on this Forum?

Even try Google for information on this!
This is not an abnormal or rarely occurring screw-up.

 

What did you install on your fwd acum?

Many will disagree, but if you rev the sht out of the tranny and it does moves, you probably installed the sonnax fwd piston and forgot to tap the 2 checkballs inside the pin bore.

Ask me how i know 🤣

 

yes I did. The results ended in forward sprag being the culprit. Only one instance of the forward accumulator piston as mentioned by Ed. But this is a stock rebuild, no fancy sonnax parts used or missing checkballs on that component.

 

factory rebuild, no sonnax parts lol or transgo
I've seen one thread of u arguing with some members here about the forward sprag vs the fwd accumulator piston lol ended up being the sprag being broken

 

Why do we still need forums for twenty plus years old cars if every answer can be found on Google?

 

I don't know who said: "every answer can be found on Google"...

But the Answer to the O.P.'s Question, can most certainly be answered via a Google Search.

We are discussing one of the Most-Common Transmissions ever produced...
and one of the Most-Common Concerns regarding that Transmission.

One of the reasons that Google is full of this Information, is due to Posts from this Forum showing-up all over Google search-results!


Well, that just about spent my tolerance for idiotic questions...
So y'all have a nice day.

 

OP I have not responded earlier because it appears as if you have checked everything that I can think of. I reread your post again this morning and it still sounds to me like the forward sprag is not holding. I remember a 4T80 that I rebuilt a long while back where I reused what looked a like good sprag that would hold when checked by hand, but when installed it would not hold. From that point on I never reused a sprag. Not saying that a used sprag was installed in this trans just that they can look good and test good on the bench and not hold once installed. I hope you have found the problem by now, if not maybe this will help.

 

“One of the reasons that Google is full of this Information, is due to Posts from this Forum showing-up all over Google search-results!
” Exactly this. When is the last time a new question has been asked? Probably something like twenty years ago right? So technically the response to every question could be, “do a search.” Who joins a forum just to use the forums “search function?” Not too many I’d bet because like you said one can use a search engine like Google and find a bunch of answers on a bunch of forums at once. So if everyone started doing that forum traffic would slow to a crawl and eventually it’d shut down. Sponsors aren’t going to sponsor a forum that only has six moderators sitting around talking about the “good ole days.”

 

I hate to update this post after so long, but I just remembered and thought it might help someone. The owner of the vehicle had crushed the harness when he first did the trans install, he mentioned it at the very end of us dealing with this vehicle. We didn't catch the issue as it passed the continuity test, but upon disassembly of the harness, it was clear that was the issue. Fixed it, and trans worked as it should.